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Callister - An introduction - 8th edition

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676 • Chapter 17 / Corrosion and Degradation of Materials<br />

Zinc<br />

Zn<br />

Zn 2+<br />

Acid solution<br />

Figure 17.1 The electrochemical reactions<br />

associated with the corrosion of zinc in an acid<br />

solution. (From M. G. Fontana, Corrosion<br />

Engineering, 3rd <strong>edition</strong>. Copyright © 1986 by<br />

McGraw-Hill Book Company. Reproduced with<br />

permission.)<br />

e – e – H +<br />

H + H 2<br />

Reduction of a<br />

multivalent metal<br />

ion to a lower<br />

valence state<br />

Reduction of a metal<br />

ion to its electrically<br />

neutral atom<br />

cathode<br />

<strong>An</strong>y metal ions present in the solution may also be reduced; for ions that can exist<br />

in more than one valence state (multivalent ions), reduction may occur by<br />

M n e ¡ M 1n12<br />

(17.6)<br />

in which the metal ion decreases its valence state by accepting an electron. Or a<br />

metal may be totally reduced from an ionic to a neutral metallic state according to<br />

M n ne ¡ M<br />

(17.7)<br />

The location at which reduction occurs is called the cathode. Furthermore, it is<br />

possible for two or more of the preceding reduction reactions to occur simultaneously.<br />

<strong>An</strong> overall electrochemical reaction must consist of at least one oxidation and<br />

one reduction reaction, and will be the sum of them; often the individual oxidation<br />

and reduction reactions are termed half-reactions. There can be no net electrical<br />

charge accumulation from the electrons and ions; that is, the total rate of oxidation<br />

must equal the total rate of reduction, or all electrons generated through oxidation<br />

must be consumed by reduction.<br />

For example, consider zinc metal immersed in an acid solution containing H <br />

ions. At some regions on the metal surface, zinc will experience oxidation or corrosion<br />

as illustrated in Figure 17.1, and according to the reaction<br />

Zn ¡ Zn 2 2e <br />

(17.8)<br />

Because zinc is a metal, and therefore a good electrical conductor, these electrons may<br />

be transferred to an adjacent region at which the H ions are reduced according to<br />

2H 2e ¡ H 2 1gas2<br />

(17.9)<br />

If no other oxidation or reduction reactions occur, the total electrochemical reaction<br />

is just the sum of reactions 17.8 and 17.9, or<br />

Zn ¡ Zn 2 2e <br />

2H 2e ¡ H 2 1gas2<br />

Zn 2H ¡ Zn 2 H 2 1gas2<br />

(17.10)<br />

<strong>An</strong>other example is the oxidation or rusting of iron in water, which contains<br />

dissolved oxygen. This process occurs in two steps; in the first, Fe is oxidized to Fe 2<br />

[as Fe(OH) 2 ],<br />

Fe 1 2O 2 H 2 O ¡ Fe 2 2OH ¡ Fe1OH2 2<br />

(17.11)

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